The abc of clinical genetics & Moleculer

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The abc of clinical genetics & Moleculer

  1. 1. ABCOF CLINICAL GENETICS Helen M Kingston Third edition ABCOFCLINICALGENETICSTHIRDEDITIONKingston Primary Care This ever popular introduction to clinical genetics has been extensively rewritten and enlarged to reflect the enormous advances that have been made in recent years. New information is included in this edition on: • genetic services • genetic assessment and counselling • single gene disorders • cancer genetics • DNA technology and molecular analysis • gene therapy • the internet and human genetics This is an ideal basic text on clinical genetics. It covers all the issues that family doctors, obstetricians, paediatricians and other practitioners need to know, and are likely to be asked by families, from the scientific basis of inheritance to discussion of the specific disorders. Using the winning ABC formula of concise explanation enhanced with extensive illustrations and written by authoritative workers in the medical genetics field, this is an invaluable reference that is relevant worldwide. Related titles from BMJ Books ABC of Antenatal Care ABC of Labour Care ABC of the First Year www.bmjbooks.com Visit our web site: www.bmjbooks.com ABCOF CLINICAL GENETICS Helen M Kingston 40821 ABC of Clinical Genetics 8/11/01 11:01 AM Page 1
  2. 2. ABC OF CLINICAL GENETICS, THIRD EDITION
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  4. 4. ABC OF CLINICAL GENETICS Third edition Helen M Kingston Consultant Clinical Geneticist, Regional Genetic Service, St Mary’s Hospital, Manchester, UK
  5. 5. © BMJ Books 2002 BMJ Books is an imprint of the BMJ Publishing Group All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording and/or otherwise, without the prior written permission of the publishers First published 1989 Second impression (revised) 1990 Second edition 1994 Second impression (revised) 1997 Third impression 1999 Third edition 2002 by BMJ Books, BMA House, Tavistock Square, London WC1H 9JR www.bmjbooks.com Cover image depicts a computer representation of the beta DNA molecule. Produced with permission from Prof K Seddon and Dr T Evans, Queen’s University, Belfast/Science Photo Library. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0-7279-1627-0 Typeset by Newgen Imaging Systems (P) Ltd., Chennai, India Printed in Malaysia by Times Offset
  6. 6. v Contents Contributors vi Preface vii 1. Clinical genetic services 1 2. Genetic assessment 5 3. Genetic counselling 8 4. Chromosomal analysis 14 5. Common chromosomal disorders 18 6. Mendelian inheritance 25 7. Unusual inheritance mechanisms 30 8. Estimation of risk in mendelian disorders 35 9. Detection of carriers 39 10. Single gene disorders 45 11. Genetics of cancer 56 12. Genetics of common disorders 63 13. Dysmorphology and teratogenesis 68 14. Prenatal diagnosis 73 15. DNA structure and gene expression 78 16. Gene mapping and molecular pathology 82 17. Techniques of DNA analysis 88 18. Molecular analysis of mendelian disorders 94 19. Treatment of genetic disorders 99 20. The internet and human genetics 104 Websites 106 Glossary 108 Further reading list 112 Index 114
  7. 7. Contributors David Gokhale Scientist, Molecular Genetic Laboratory, Regional Genetic Service, St Mary’s Hospital, Manchester Lauren Kerzin-Sturrar Principal Genetic Associate, Regional Genetic Service, St Mary’s Hospital, Manchester Tara Clancy Senior Genetic Associate, Regional Genetic Service, St Mary’s Hospital, Manchester Bronwyn Kerr Consultant Clinical Geneticist, Regional Genetic Service, St Mary’s Hospital, Manchester vi
  8. 8. Preface Since the first edition of this book in 1989 there have been enormous changes in clinical genetics, reflecting the knowledge generated from the tremendous advances in molecular biology, culminating in the publication of the first draft of the human genome sequence in 2001, and the dissemination of information via the internet. The principles of genetic assessment and the aims of genetic counselling have not changed, but the classification of genetic disease and the practice of clinical genetics has been significantly altered by this new knowledge. To interpret all the information now available it is necessary to understand the basic principles of inheritance and its chromosomal and molecular basis. Recent advances in medical genetics have had a considerable impact on other specialties, providing a new range of molecular diagnostic tests applicable to many branches of medicine, and more patients are presenting to their general practitioners with concerns about a family history of disorders such as cancer. Increasingly, other specialties are involved in the genetic aspects of the conditions they treat and need to provide information about genetic risk, undertake genetic testing and provide appropriate counselling. All medical students, irrespective of their eventual career choice therefore need to be familiar with genetic principles, both scientific and clinical, and to be aware of the ethical implications of genetic technologies that enable manipulation of the human genome that may have future application in areas such as gene therapy of human cloning. The aim of this third edition of the ABC is therefore to provide an introduction to the various aspects of medical genetics for medical students, clinicians, nurses and allied professionals who are not working within the field of genetics, to generate an interest in the subject and to guide readers in the direction of further, more detailed information. In producing this edition, the chapters on molecular genetics and its application to clinical practice have been completely re-written, bringing the reader up to date with current molecular genetic techniques and tests as they are applied to inherited disorders. An introduction to the internet in human genetics has also been included. There are new chapters on genetic services, genetic assessment and genetic counselling together with a new chapter highlighting the clinical and genetic aspects of some of the more common single gene disorders. Substantial alterations have been made to most other chapters so that they reflect current practice and knowledge, although some sections of the previous edition remain. A glossary of terms is included for readers who are not familiar with genetic terminology, a further reading list is incorporated and a list of websites included to enable access to data that is changing on a daily basis. As in previous editions, illustrations are a crucial component of the book, helping to present complex genetic mechanisms in an easily understood manner, providing photographs of clinical disorders, tabulating genetic diseases too numerous to be discussed individually in the text and showing the actual results of cytogenetic and molecular tests. I am grateful to many colleagues who have helped me in producing this edition of the ABC. In particular, I am indebted to Dr David Gokhale who has re-written chapters 17, 18 and 20, and has provided the majority of the illustrations for chapters 16, 17 and 18. I am also grateful to Lauren Kerzin-Storrar and Tara Clancy for writing chapter 3 and to Dr Bronwyn Kerr for contributing to chapter 11. Numerous colleagues have provided illustrations and are acknowledged throughout the book. In particular, I would like to thank Professor Dian Donnai, Dr Lorraine Gaunt and Dr Sylvia Rimmer who have provided many illustrations for this as well as previous editions, and to Helena Elliott who has prepared most of the cytogenetic pictures incorporated into this new edition. I am also very grateful to the families who allowed me to publish the clinical photographs that are included in this book to aid syndrome recognition. Helen M Kingston vii
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  10. 10. Development of medical genetics The speciality of medical genetics is concerned with the study of human biological variation and its relationship to health and disease. It encompasses mechanisms of inheritance, cytogenetics, molecular genetics and biochemical genetics as well as formal, statistical and population genetics. Clinical genetics is the branch of the specialty involved with the diagnosis and management of genetic disorders affecting individuals and their families. Genetic counselling clinics were first established in the USA in 1941 and in the UK in 1946. Some of the disorders dealt with in these early clinics were ones that are seldom referred today, such as skin colour, eye colour, twinning and rhesus haemolytic disease. Other referrals were very similar to those being seen today – namely, mental retardation, neural tube defects and Huntington disease. Prior to the inception of these clinics, the patterns of dominant and recessive inheritance, described by Mendel in 1865, were recognised in human disorders. Autosomal recessive inheritance of alkaptonuria had been recognised in 1902 by Archibald Garrod, who also introduced the term “inborn errors of metabolism”. In 1908, the Hardy–Weinberg principle of population genetics was delineated and remains the basis of calculating carrier frequencies for autosomal recessive disorders. The term, “genetic counselling” was introduced by Sheldon Reed, whose definition of the process is given later in this chapter. DNA, initially called “nuclein”, had been discovered by Meischer in 1867 and the first illustration of human chromosomes was published by Walther Fleming in 1882 although the term “chromosome” was not coined until 1888 and the chromosomal basis of mendelism only proposed in 1903. The correct chromosome number in humans was not established until 1956 and the association between trisomy 21 and Down syndrome was reported in 1959. The structure of DNA was determined by Watson and Crick in 1953 and by 1966 the complete genetic code had been cracked. DNA analysis became possible during the 1970s with the discovery of restriction endonucleases and development of the Southern blotting technique. These advances have led to the mapping and isolation of many genes and subsequent mutation analysis. Enormous advances in molecular biology techniques have resulted in publication of the draft sequence of the human genome in 2000. As a result of these scientific discoveries and developments, clinical geneticists are able to use chromosomal analysis and molecular genetic tests to diagnose many genetic disorders. Genetic disease Genetic disorders place considerable health and economic burdens not only on affected individuals and their families but also on the community. As more environmental diseases are successfully controlled those that are wholly or partly genetically determined are becoming more important. Despite a general fall in the perinatal mortality rate, the incidence of lethal malformations in newborn infants remains constant. Between 2 and 5% of all liveborn infants have genetic disorders or congenital malformations. These disorders have been estimated to account for one third of admissions to paediatric wards, and they contribute appreciably to perinatal and childhood mortality. Many common diseases in adult life also 1 1 Clinical genetic services Figure 1.1 Gregor Mendel 1822–84 Table 1.1 Prevalence of genetic disease Estimated prevalence Type of genetic disease per 1000 population Single gene Autosomal dominant 2–10 Autosomal recessive 2 X linked recessive 1–2 Chromosomal abnormalities 6–7 Common disorders with appreciable 7–10 genetic component Congenital malformations 20 Total 38–51 Figure 1.2 Archibald Garrod 1858–1936 Figure 1.3 The discoverers of the structure of DNA. James Watson (b. 1928) at left and Francis Crick (b. 1916), seen with their model of part of a DNA molecule in 1953 (with permission from A Barrington Brown/Science Photo Library)
  11. 11. have a considerable genetic predisposition, including coronary heart disease, diabetes and cancer. Though diseases of wholly genetic origin are individually rare, they are numerous (several thousand single gene disorders are described) and are therefore important. Genetic disorders are incurable and often severe. Some are amenable to treatment but many are not, so that the emphasis is often placed on prevention, either of recurrence within an affected family, or of complications in a person who is already affected. Increasing awareness, both within the medical profession and in the general population, of the genetic contribution to disease and the potential implications of a positive family history, has led to an increasing demand for specialist clinical genetic services. Some aspects of genetics are well established and do not require referral to a specialist genetics clinic – for example, the provision of amniocentesis to exclude Down syndrome in pregnancies at increased risk. Other aspects are less well understood – for example the application of molecular genetic analysis in clinical practice, which is an area of rapidly advancing technology requiring the facilities of a specialised genetics centre. Organisation of genetic services In the UK, NHS genetic services are provided in integrated regional centres based in teaching hospitals, incorporating clinical and laboratory departments usually in close liaison with academic departments of human genetics. Clinical genetics Clinical services are provided by consultant clinical geneticists, specialist registrars and genetic associates (nurses or graduates with specialist training in genetics and counselling). Most clinical genetic departments provide a “hub and spoke” service, undertaking clinics in district hospitals as well as at the regional centre. Patients referred to the genetic clinic are contacted initially by the genetic associate and many are visited at home before attending the clinic. The purpose of the home visit is to explain the nature of the genetic clinic appointment, determine the issues of importance to the family and obtain relevant family history information. The genetic associate is usually present at the clinic appointment and participates in the counselling process with the clinical geneticist. At the clinic appointment genetic investigations may be instituted to establish or confirm a diagnosis and information is given to the individual or family about the condition regarding diagnosis, prognosis, investigation, management and genetic consequences. Written information is usually provided after the clinic appointment so that the family have a record of the various aspects discussed. After the appointment, follow-up visits at home or in the clinic are arranged as necessary. The genetic associate plays an important role in liaising with primary care and other agencies involved with the family. There are a wide variety of reasons leading to referral to the genetic clinic. The referral may be for diagnosis in cases where a genetic disorder is suspected; for counselling when a genetic condition has been identified; for genetic investigation of family members when there is a family history of an inherited disorder; or for information regarding prenatal diagnosis. The disorders seen include sporadic birth defects and chromosomal syndromes as well as mendelian, mitochondrial and multifactorial conditions. Specialist or multidisciplinary clinics are provided by some genetic centres, such as for dysmorphology, inherited cancers, neuromuscular disorders, Huntington disease, Marfan syndrome, ophthalmic disorders or hereditary deafness. ABC of Clinical Genetics 2 Box 1.1 Type of genetic disease Single gene (mendelian) • Numerous though individually rare • Clear pattern of inheritance • High risk to relatives Multifactorial • Common disorders • No clear pattern of inheritance • Low or moderate risk to relatives Chromosomal • Mostly rare • No clear pattern of inheritance • Usually low risk to relatives Somatic mutation • Accounts for mosaicism • Cause of neoplasia Box 1.2 Common reasons for referral to a genetic clinic • Children with congenital abnormalities (birth defects), learning disability, dysmorphic features • Children with chromosomal disorders or inherited conditions • Adults affected by congenital abnormality or an inherited condition • Adults known to carry or at risk of carrying, a balanced chromosomal rearrangement • Couples who have lost a child or stillborn baby with a congenital abnormality or inherited condition • Couples who have suffered reproductive loss (termination of pregnancy for fetal abnormality or recurrent miscarriage) • Pregnant women and their partners, when fetal abnormality is detected • Children and adults with a family history of a known genetic disorder • Adults at risk of developing an inherited condition who may request predictive testing • Couples who may transmit a genetic condition to their children • Individuals with a family history of a common condition with a strong genetic component, including familial cancers Figure 1.4 Explaining genetic mechanisms and risks during genetic counselling
  12. 12. Information about clinical genetic centres in the UK can be obtained from the British Society for Human Genetics (incorporating the Clinical Genetics Society and the Association of Genetic Nurses) website at www.bshg.org.uk Cytogenetics Cytogenetic laboratories undertake chromosomal analysis on a variety of samples including whole blood (collected into lithium heparin), amniotic fluid, chorion villus or placental samples, cultures of solid tissues and bone marrow aspirates. Analysis is undertaken to diagnose chromosomal disorders when a diagnosis is suspected clinically, to identify carriers of familial chromosomal rearrangements when there is a family history and to provide information related to therapy and prognosis in certain neoplastic conditions. Some of the main indications for performing chromosomal analysis are listed in the box. Routine chromosomal analysis requires the study of metaphase chromosomes in cultured cells. Results are usually available in 1–3 weeks. Molecular genetic analysis by fluorescence in situ hydridisation (FISH) studies is possible for certain conditions. These studies are usually performed on cultured cells, but in some cases (such as urgent prenatal confirmation of trisomy 21) rapid results may be obtained by analysis of interphase nuclei in uncultured cells. Information about cytogenetic centres in the UK can be obtained from the Association of Clinical Cytogeneticists (ACC) website at www.acc.org.uk Molecular genetics Molecular genetic laboratories offer a range of DNA tests. Direct mutation analysis is available for certain conditions and provides confirmation of clinical diagnosis in affected individuals, presymptomatic diagnosis for individuals at risk of specific conditions, carrier detection and prenatal diagnosis. Mutation analysis for rare disorders is usually undertaken on a supra-regional or national basis in designated laboratories. For mendelian disorders in which mutation analysis is not possible, gene tracking using linked DNA markers may be used to predict inheritance of certain conditions (for example Marfan syndrome and Neurofibromatosis type 1) if the family structure is suitable. DNA can be extracted from any tissue containing nucleated cells, including stored tissue blocks. Tests are usually performed on whole blood collected into EDTA anticoagulant, or buccal samples obtained by scraping the inside of the cheek or by mouth wash. Once extracted, frozen DNA samples can be stored indefinitely. Samples can therefore be collected from family members and stored for future analysis of disorders that are currently not amenable to molecular analysis. In the UK, the Clinical Molecular Genetics Society (CMGS) provides data on molecular services offered by individual laboratories through their website at www.cmgs.org.uk Biochemical genetics Specialised biochemical genetic departments offer clinical and laboratory services for a range of inherited metabolic disorders. Routine neonatal screening for conditions such as phenylketonuria (PKU) and congenital hypothyroidism are undertaken on neonatal blood samples taken from all newborn babies. Investigations performed on children presenting with other metabolic disorders are carried out on a range of samples including urine, blood, CSF, cultured fibroblasts and muscle biopsies. Tests are undertaken to identify conditions such as disorders of amino acids, organic acids and mucopolysaccharides, lysosomal and lipid storage diseases, and Clinical genetic services 3 Box 1.3 Common reasons for cytogenetic analysis Postnatal • Newborn infants with birth defect • Children with learning disability • Children with dysmorphic features • Familial chromosomal rearrangement in relative • Infertility • Recurrent miscarriages Prenatal • Abnormalities on ultrasound scan • Increased risk of Down syndrome (maternal age or biochemical screening) • Previous child with a chromosomal abnormality • One parent carries a structural chromosomal abnormality Box 1.4 Some common reasons for molecular genetic analysis • Cystic fibrosis • Haemoglobinopathies • Duchenne and Becker muscular dystrophy • Myotonic dystrophy • Huntington disease • Fragile X syndrome • Spinal muscular atrophy • Spinocerebellar ataxia • Hereditary neuropathy (Charcot-Marie-Tooth) • Familial breast cancer (BRCA 1 and 2) • Familial adenomatous polyposis Figure 1.5 Conventional cytogenic analysis using light microscopy Figure 1.6 Typical molecular genetics laboratory
  13. 13. peroxisomal and mitochondrial disorders. Tests for other metabolites or enzymes are performed when a diagnosis of a specific disorder is being considered. In the UK, the Society for the Study of Inborn Errors of Metabolism publishes information on centres providing biochemical genetic tests. Their website address is www.ssiem.org.uk Genetic registers Genetic registers have been in use in the UK for about 30 years and most genetic centres operate some type of register for specified disorders. In some cases the register functions as a reference list of cases for diagnostic information, but generally the system is used to facilitate patient management. Ascertainment of cases is usually through referrals made to the genetic centre. Less often there is an attempt to actively ascertain all affected cases within a given population. To function effectively most registers contain information about relatives at risk as well as affected individuals and may contain information from genetic test results. Establishment of a register enables long-term follow up of family members. This is important for children at risk who may not need counselling and investigation for many years. A unique aspect of a family based genetic register is that it includes clinically unaffected individuals who may require continued surveillance and enables continued contact with couples at risk of transmitting disorders to their children. Registers are particularly useful for disorders amenable to DNA analysis in which advances of clinical importance are likely to improve future genetic testing and where families will need to be updated with new information. Disorders suited to a register approach include dominant disorders with late onset such as Huntington disease and myotonic dystrophy where pre-symptomatic diagnosis may be requested by some family members or health surveillance is needed by affected individuals; X linked disorders such as Duchenne and Becker muscular dystrophy where carrier testing is offered to female relatives, and chromosomal translocations where relatives benefit from carrier testing. Registers can also provide data on the incidence and natural course of disease as well as being used to monitor the provision and effectiveness of services. Genetic register information is held on computer and is subject to the Data Protection Act. No one has his/her details included on a register without giving informed consent. ABC of Clinical Genetics 4 Clinical geneticist Laboratory services Genetic nurse or counsellor Secondary and tertiary care services Primary care services FAMILY Patient support groups Figure 1.8 Interactions between families with genetic disorders and various medical and support services Figure 1.7 Amino acid analyser in biochemical genetics laboratory
  14. 14. Genetic diagnosis The role of clinical geneticists is to establish an accurate diagnosis on which to base counselling and then to provide information about prognosis and follow up, the risk of developing or transmitting the disorder, and the ways in which this may be prevented or ameliorated. Throughout, the family requires support in adjusting to the implications of genetic disease and the consequent decisions that may have to be made. History taking Diagnosis of genetic disorders is based on taking an accurate history and performing clinical examination, as in any other branch of medicine. The history and examination will focus on aspects relevant to the presenting complaint. When a child presents with birth defects, for example, information needs to be gathered concerning parental age, maternal health, pregnancy complications, exposure to potential teratogens, fetal growth and movement, prenatal ultrasound scan findings, mode of delivery and previous pregnancy outcomes. Information regarding similar or associated abnormalities present in other family members should also be sought. In conditions with onset in adult life, the age at onset, mode of presentation and course of the disease in affected relatives should be documented, together with the ages reached by unaffected relatives. Examination Thorough physical examination is required, but emphasis will be focused on relevant anatomical regions or body systems. Detailed examination of children with birth defects or dysmorphic syndromes is crucial in attempting to reach a diagnosis. A careful search should be made for both minor and major congenital abnormalities. Measurements of height, weight and head circumference are important and standard growth charts and tables are available for a number of specific conditions, such as Down syndrome, Marfan syndrome and achondroplasia. Other measurements, including those of body proportion and facial parameters may be appropriate and examination findings are often best documented by clinical photography. In some cases, clinical geneticists will need to rely on the clinical findings of other specialists such as ophthalmologists, neurologists and cardiologists to complete the clinical evaluation of the patient. The person attending the clinic may not be affected, but may be concerned to know whether he or she might develop a particular disorder or transmit it to any future children. In such cases, the diagnosis in the affected relative needs to be clarified, either by examination or by review of relevant hospital records (with appropriate consent). Apparently unaffected relatives should be examined carefully for minor or early manifestations of a condition to avoid inappropriate reassurance. In myotonic dystrophy, for example, myotonia of grip and mild weakness of facial muscles, sterno-mastoids and distal muscles may be demonstrated in asymptomatic young adults and indicate that they are affected. Subjects who may show signs of a late onset disorder should be examined before any predictive genetic tests are done, so that the expectation of the likely result is realistic. Some young adults who request predictive tests to reassure themselves that they are not affected may not wish to proceed with definitive tests if they are told that their clinical examination is not entirely normal. 5 2 Genetic assessment Figure 2.1 Recording family history details by drawing a pedigree Figure 2.2 The presence of one congenital anomaly should prompt a careful search for other anomalies Figure 2.4 Growth chart showing typical heights in Marfan syndrome and Achondroplasia compared to normal centiles WEIGHT kg years 40 35 30 25 20 15 10 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 5 6 7 8 9 10 11 12 13 14 15 16 17 5 6 7 8 9 10 11 12 13 14 15 16 17 200 155 160 165 170 175 180 185 190 195 200 115 110 105 100 95 90 105 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 HEIGHT cm 5-18 yrs With provision for school reception class NAME 99.6th 98th 91st 75th 50th 25th 9th 2nd 0.4th 99.6th 98th 91st 75th 50th 25th 9th 2nd 0.4th Marfan syndrome Achondroplasia Figure 2.3 Physical measurements are an important part of clinical examination
  15. 15. Investigations Investigation of affected individuals and family members may include conventional tests such as x-rays and biochemical analysis as well as cytogenetic and molecular genetic tests. A search for associated anomalies in children with chromosomal disorders often includes cranial, cardiac and renal imaging along with tests for other specific components of the particular syndrome, such as immune deficiency. In some genetic disorders affected individuals may require regular investigations to detect disease-associated complications, such as cardiac arrhythmias and reduced lung function in myotonic dystrophy. Screening for disease complications in asymptomatic relatives at risk of a genetic disorder may also be appropriate, for example, 24-hour urine catecholamine estimation and abdominal scans for individuals at risk of von Hippel–Lindau disease. Drawing a pedigree Accurate documentation of the family history is an essential part of genetic assessment. Family pedigrees are drawn up and relevant medical information on relatives sought. There is some variation in the symbols used for drawing pedigrees. Some suggested symbols are shown in the figure. It is important to record full names and dates of birth of relatives on the pedigree, so that appropriate hospital records can be obtained if necessary. Age at onset and symptoms in affected relatives should be documented. Specific questions should be asked about abortions, stillbirth, infant death, multiple marriages and consanguinity as this information may not always be volunteered. When a pedigree is drawn, it is usually easiest to start with the person seeking advice (the consultand). Details of first degree relatives (parents, siblings and children) and then second degree relatives (grandparents, aunts, uncles, nieces and nephews) are added. If indicated, details of third degree relatives can be added. If the consultand has a partner, a similar pedigree is constructed for his or her side of the family. The affected person (proband) through whom the family has been ascertained is usually indicated by an arrow. Confirmation of a clinical diagnosis may identify a defined mode of inheritance for some conditions. In others, similar phenotypes may be due to different underlying mechanisms, for example, limb girdle muscular dystrophy may follow dominant or recessive inheritance and the pedigree may give clues as to which mechanism is more likely. In cases where no clinical diagnosis can be reached, information on genetic risk can be given if the pedigree clearly indicates a particular mode of inheritance. However, when there is only a single affected individual in the family, recurrence risk is difficult to quantify if a clinical diagnosis cannot be reached. Estimation of risk For single gene disorders amenable to mutation analysis, risks to individuals of developing or transmitting particular conditions can often be identified in absolute terms. In many conditions, however, risks are expressed in terms of probabilities calculated from pedigree data or based on empirical risk figures. An important component of genetic counselling is explaining these risks to families in a manner that they can understand and use in decision making. Mendelian disorders due to mutant genes generally carry high risks of recurrence whereas chromosomal disorders generally have a low recurrence risk. For many common conditions there is no clearly defined pattern of inheritance ABC of Clinical Genetics 6 Unaffected male, female, sex unknown Clinically affected Multiple traits Proband Consultand Deceased (age at death) Carrier of autosomal or X-linked recessive trait who will not become affected Presymptomatic carriers who may manifest disease later Number of siblings Partners separated Consanguinity Children Ongoing pregnancy miscarriage, termination Stillbirth (gestation) Twins dizygous, monozyous No children 2 3 P SB 32wk d.63y Figure 2.6 Pedigree symbols Figure 2.5 Supravalvular aortic stenosis in a child with William syndrome Figure 2.7 Hand drawn pedigree of a family with Duchenne muscular dystrophy identifying obligate carriers and other female relatives at risk
  16. 16. and the empirical figures for risk of recurrence are based on information derived from family studies. There is considerable heterogeneity observed in many genetic disorders. Similar phenotypes may be due to mutations at different loci (locus heterogeneity) or to different modes of inheritance. In autosomal recessive deafness there is considerable locus heterogeneity with over 30 different loci known to cause non- syndromic severe congenital deafness. The risk to offspring of two affected parents will be 100% if their deafness is due to gene mutations at the same locus, but negligible if due to gene mutations at different loci. In some disorders, for example hereditary spastic paraplegia and retinitis pigmentosa, autosomal dominant, autosomal recessive and X linked recessive inheritance have been documented. Definite recurrence risks cannot be given if there is only one affected person in the family, since dominant and recessive forms cannot be distinguished clinically. Perception of risk is affected by the severity of the disorder, its prognosis and the availability of treatment or palliation. All these aspects need to be considered when information is given to individuals and families. The decisions that couples make about pregnancy are influenced partly by the risk of transmitting the disorder, and partly by its severity and the availability of prenatal diagnosis. A high risk of a mild or treatable disorder may be accepted, whereas a low risk of a severe disorder can have a greater impact on reproductive decisions. Conversely, where no prenatal diagnosis is possible, a high risk may be more acceptable for a lethal disorder than for one where prolonged survival with severe handicap is expected. Moral and religious convictions play an important role in an individual’s decision making regarding reproductive options and these beliefs must be respected. Consanguinity Consanguinity is an important issue to identify in genetic assessment because of the increased risk of autosomal recessive disorders occurring in the offspring of consanguineous couples. Everyone probably carries at least one harmful autosomal recessive gene. In marriages between first cousins the chance of a child inheriting the same recessive gene from both parents that originated from one of the common grandparents is 1 in 64. A different recessive gene may similarly be transmitted from the other common grandparent, so that the risk of homozygosity for a recessive disorder in the child is 1 in 32. If everyone carries two recessive genes, the risk would be 1 in 16. Marriage between first cousins generally increases the risk of severe abnormality and mortality in offspring by 3–5% compared with that in the general population. The increased risk associated with marriage between second cousins is around 1%. Marriage between first and second degree relatives is almost universally illegal, although marriages between uncles and nieces occur in some Asian countries. Marriage between third degree relatives (between cousins or half uncles and nieces) is more common and permitted by law in many countries. The offspring of incestuous relationships are at high risk of severe abnormality, mental retardation and childhood death. Only about half of the children born to couples who are first degree relatives are normal and this has important implications for decisions about termination of pregnancy or subsequent adoption. Genetic assessment 7 Degree of genetic relationship Second: First: Third: Fourth: Fifth: Parent–child Siblings Uncle–niece Half siblings Double first cousins First cousins Half-uncle–niece Second cousins First cousins once removed Proportion of genes shared 1/2 1/4 1/8 1/16 1/32 Example Figure 2.9 Proportion of genes shared by different relatives Figure 2.8 The same pedigree as Figure 2.7 drawn using Cyrillic computer software (Cherwell Scientific Publishing)
  17. 17. Genetic counselling has been defined as a communication process with both educative and psychotherapeutic aims. While genetic counselling must be based on accurate diagnosis and risk assessment, its use by patients and families will depend upon the way in which the information is given and its psychosocial impact addressed. The ultimate aim of genetic counselling is to help families at increased genetic risk to live and reproduce as normally as possible. While genetic counselling is a comprehensive activity, the particular focus will depend upon the family situation. A pregnant couple at high genetic risk may need to make urgent decisions concerning prenatal diagnosis; parents of a newly diagnosed child with a rare genetic disorder may be desperate for further prognostic information, while still coming to terms with the diagnosis; a young adult at risk of a late onset degenerative disorder may be well informed about the condition, but require ongoing discussions about whether to go ahead with a presymptomatic test; and a teenage girl, whose brother has been affected with an X linked disorder, may be apprehensive to learn about the implication for her future children, and unsure how to discuss this with her boyfriend. Being able to establish the individual’s and the family’s particular agenda, to present information in a clear manner, and to address psychosocial issues are all crucial skills required in genetic counselling. Psychosocial issues The psychosocial impact of a genetic diagnosis for affected individuals and their families cannot be over emphasised. The diagnosis of any significant medical condition in a child or adult may have psychological, financial and social implications, but if the condition has a genetic basis a number of additional issues arise. These include guilt and blame, the impact on future reproductive decisions and the genetic implications to the extended family. Guilt and blame Feelings of guilt arise in relation to a genetic diagnosis in the family in many different situations. Parents very often express guilt at having transmitted a genetic disorder to their children, even when they had no previous knowledge of the risk. On the other hand, parents may also feel guilty for having taken the decision to terminate an affected pregnancy. Healthy members of a family may feel guilty that they have been more fortunate than their affected relatives and at-risk individuals may feel guilty about imposing a burden onto their partner and partner’s family. Although in most situations the person expressing guilt will have played no objective causal role, it is important to allow him or her to express these concerns and for the counsellor to reinforce that this is a normal human reaction to the predicament. Blame occurs perhaps less often than anticipated by families. Although parents often fear that their children will blame them for their adverse genetic inheritance, in practice this happens infrequently and usually only when the parents have knowingly withheld information about the genetic risk. Blame can sometimes occur in families where only one member of a couple carries the genetic risk (“It wasn’t our side”), but 8 3 Genetic counselling Box 3.1 Definition and aims of genetic counselling Genetic counselling is a communication process that deals with the human problems associated with the occurrence or risk of occurrence, of a genetic disorder in a family. The process aims to help the individual or family to: understand: • the diagnosis, prognosis and available management • the genetic basis and chance of recurrence • the options available (including genetic testing) choose: • the course of action appropriate to their personal and family situation adjust: • to the psychosocial impact of the genetic condition in the family. Adapted from American Society of Human Genetics, 1975 Figure 3.1 Explanation of genetic mechanisms is an important component of genetic counselling Figure 3.2 Impact of genetic diagnosis health and reproductive implications for extended family health and future reproductive implications for siblings Reproductive implications fo r parents I m pact of child's prognosi s Genetic diagnosis
  18. 18. again this is less likely to occur when the genetic situation has been explained and is understood. Reproductive decision making Couples aware of an increased genetic risk to their offspring must decide whether this knowledge will affect their plans for a family. Some couples may be faced with a perplexing range of options including different methods of prenatal diagnosis and the use of assisted reproductive technologies. For others the only available option will be to choose between taking the risk of having an affected child and remaining childless. Couples may need to reconsider these choices on repeated occasions during their reproductive years. Most couples are able to make reproductive choices and this is facilitated through access to full information and counselling. Decision making may be more difficult in particular circumstances, including marital disagreement, religious or cultural conflict, and situations where the prognosis for an affected child is uncertain. For many genetic disorders with variable severity, although prenatal diagnosis can be offered, the clinical prognosis for the fetus cannot be predicted. When considering reproductive decisions, it can also be difficult for a couple to reconcile their love for an affected child or family member, with a desire to prevent the birth of a further affected child. Impact on the extended family The implications of a genetic diagnosis usually reverberate well beyond the affected individual and his or her nuclear family. For example, the parents of a boy just diagnosed with Duchenne muscular dystrophy will not only be coming to terms with his anticipated physical deterioration, but may have concerns that a younger son could be affected and that daughters could be carriers. They also face the need to discuss the possible family implications with the mother’s sisters and female cousins who may already be having their own children. This is likely to be distressing even when family relationships are intact, but will be further complicated in families where relationships are less good. Family support can be very important for people coping with the impact of a genetic disorder. When there are already several affected and carrier individuals in a family, the source of support from other family members can be compromised. For some families affected by disorders such as Huntington disease and familial breast cancer (BRCA 1 and 2), a family member in need of support may be reluctant to burden relatives who themselves are coping with the disease or fears about their own risks. They may also be hesitant to discuss decisions about predictive or prenatal testing with relatives who may have made different choices themselves. The need for an independent friend or counsellor in these situations is increased. Bereavement Bereavement issues arise frequently in genetic counselling sessions. These may pertain to losses that have occurred recently or in the past. A genetic disorder may lead to reproductive loss or death of a close family member. The grief experienced after termination of pregnancy following diagnosis of abnormality is like that of other bereavement reactions and may be made more intense by parents’ feelings of guilt. After the birth of a baby with congenital malformations, parents mourn the loss of the imagined healthy child in addition to their sadness about their child’s disabilities, and this chronic sorrow may be ongoing throughout the affected child’s life. Genetic counselling 9 Box 3.3 Lay support groups Contact a Family Produces comprehensive directory of individual conditions and their support groups in the UK http: //www.cafamily.org.uk Genetic Interest Group Alliance of lay support groups in the UK which provides information and presents a unified voice for patients and families, in social and political forums http: //www.gig.org.uk Antenatal Results and Choices (ARC) Publishes and distributes an invaluable booklet for parents facing the decision whether or not to terminate a pregnancy after diagnosis of abnormality, and offers peer telephone support http: //www.cafamily.org.uk/Direct/f26html Unique Support group for individuals with rare chromosome disorders and their families http: //www.rarechromo.org European Alliance of Genetic Support Groups A federation of support groups in Europe helping families with genetic disorders http: //www.ghq-ch.com/eags The Genetic Alliance An umbrella organisation in the US representing individual support groups and aimed at helping all individuals and families with genetic disorders http: //www.geneticalliance.org National Organisation for Rare Disorders (NORD) A federation of voluntary groups in the US helping people with rare conditions http: //www.rarediseases.org Box 3.2 Possible reproductive options for those at increased genetic risk Pregnancy without prenatal diagnosis • Take the risk • Limit family size Pregnancy with prenatal diagnosis • Chorion villus sampling • Amniocentesis • Ultrasound Donor sperm • Male partner has autosomal dominant disorder • Male partner has chromosomal abnormality • Both partners carriers for autosomal recessive disorder Donor egg • Female partner carrier for X linked disorder • Female partner has autosomal dominant disorder • Female partner has chromosomal abnormality • Both partners carriers for autosomal recessive disorder Preimplantation genetic diagnosis and IVF • Available for a small number of disorders Contraception • Couples who chose to have no children • Couples wanting to limit family size • Couples waiting for new advances Sterilisation • Couples whose family is complete • Couples who chose to have no children Fostering and adoption • Couples who want children, but find all the above options unacceptable
  19. 19. Long-term support Many families will require ongoing information and support following the initial genetic counselling session, whether coping with an actual diagnosis or the continued risk of a genetic disorder. This is sometimes coordinated through regional family genetic register services, or may be requested by family members at important life events including pregnancy, onset of symptoms, or the death of an affected family member. Lay support groups are an important source of information and support. In addition to the value of contact with other families who have personal experience of the condition, several groups now offer the help of professional care advisors. In the UK there is an extensive network of support groups for a large number of individual inherited conditions and these are linked through two organisations: Contact a Family (www.cofamily.co.uk) and the Genetic Interest Group (GIG)(www.gig.org.uk). Counselling around genetic testing Genetic counselling is an integral part of the genetic testing process and is required because of the potential impact of a test result on an individual and family, as well as to ensure informed choice about undergoing genetic testing. The extent of the counselling and the issues to be addressed will depend upon the type of test being offered, which may be diagnostic, presymptomatic, carrier or prenatal testing. Testing to confirm a clinical diagnosis When a genetic test is requested to confirm a clinical diagnosis in a child or adult, specialist genetic counselling may not be requested until after the test result. It is therefore the responsibility of the clinician offering the test to inform the patient (or the parents, if a child is being tested) before the test is undertaken, that the results may have genetic as well as clinical implications. Confirming the diagnosis of a genetic disorder in a child, for example, may indicate that younger siblings are also at risk of developing the disorder. For late onset conditions such as Huntington disease, it is crucial that samples sent for diagnostic testing are from patients already symptomatic, as there are stringent counselling protocols for presymptomatic testing (see below). Presymptomatic testing Genetic testing in some late onset autosomal dominant disorders can be used to predict the future health of a well individual, sometimes many decades in advance of onset of symptoms. For some conditions, such as Huntington disease, having this knowledge does not currently alter medical management or prognosis, whereas for others, such as familial breast cancer, there are preventative options available. For adult onset disorders, testing is usually offered to individuals above the age of 18. For conditions where symptoms or preventative options occur in late childhood, such as familial adenomatous polyposis, children are involved in the testing decision. Presymptomatic testing is most commonly done for individuals at 50% risk of an autosomal dominant condition. Testing someone at 25% is avoided wherever possible, as this could disclose the status of the parent at 50% risk who may not want to have this information. There are clear guidelines for provision of genetic counselling for presymptomatic testing, which include full discussion of the potential drawbacks of testing (psychological, impact on the family and financial), with ample opportunity for an individual to withdraw from testing right up until disclosure of results, and a clear plan for follow up. ABC of Clinical Genetics 10 Box 3.4 Genetic testing defined Diagnostic – confirms a clinical diagnosis in a symptomatic individual Presymptomatic (“predictive”) – confirms that an individual will develop the condition later in life Susceptibility – identifies an individual at increased risk of developing the condition later in life Carrier – identifies a healthy individual at risk of having children affected by the condition Prenatal – diagnoses an affected fetus Figure 3.3 Lay support group leaflets Genetic Counselling • One or more sessions • Molecular confirmation of diagnosis in affected relative • Discussion of clinical and genetic aspects of condition, and impact on family Patient requests test (interval of several months suggested) Pre-test Counselling • At least one session • Seen by 2 members of staff (usually clinical geneticist and genetic counsellor) • Involvement of partner encouraged • Full discussion about: • Motivation for requesting test • Alternatives to having a test • Potential impact of test result Psychological Financial Social (relationships with partner/family • Strategies for coping with result Figure 3.4 continued
  20. 20. Carrier testing Testing an individual to establish his or her carrier state for an autosomal or X linked recessive condition or chromosomal rearrangement, will usually be for future reproductive, rather than health, implications. Confirmation of carrier state may indicate a substantial risk of reproductive loss or of having an affected child. Genetic counselling before testing ensures that the individual is informed of the potential consequences of carrier testing including the option of prenatal diagnosis. In the presence of a family history, carrier testing is usually offered in the mid-teens when young people can decide whether they want to know their carrier status. For autosomal recessive conditions such as cystic fibrosis, some people may wish to wait until they have a partner so that testing can be done together, as there will be reproductive consequences only if both are found to be carriers. Prenatal testing The availability of prenatal genetic testing has enabled many couples at high genetic risk to embark upon pregnancies that they would otherwise have not undertaken. However, prenatal testing, and the associated option of termination of pregnancy, can have important psychological sequelae for pregnant women and their partners. In the presence of a known family history, genetic counselling is ideally offered in advance of pregnancy so that couples have time to make a considered choice. This also enables the laboratory to complete any family testing necessary before a prenatal test can be undertaken. Counselling should be provided within the antenatal setting when prenatal genetic tests are offered to couples without a previous family history, such as amniocentesis testing after a raised Down syndrome biochemical screening result. To help couples make an informed choice, information should be presented about the condition, the chance of it occurring, the test procedure and associated risks, the accuracy of the test, and the potential outcomes of testing including the option of termination of pregnancy. Couples at high genetic risk often require ongoing counselling and support during pregnancy. Psychologically, many couples cope with the uncertainty by remaining tentative about the pregnancy until receiving the test result. If the outcome of testing leads to termination of a wanted pregnancy, follow-up support should be offered. Even if favourable results are given, couples may still have some anxiety until the baby is born and clinical examination in the newborn period gives reassurance about normality. Occasionally, confirmatory investigations may be indicated. Legal and ethical issues There are many highly publicised controversies in genetics, including the use of modern genetic technologies in genetic testing, embryo research, gene therapy and the potential application of cloning techniques. In everyday clinical practice, however, the legal and ethical issues faced by professionals working in clinical genetics are generally similar to those in other specialities. Certain dilemmas are more specific to clinical genetics, for example, the issue of whether or not genetic information belongs to the individual and/or to other relatives remains controversial. Public perception of genetics is made more sensitive by past abuses, often carried out in the name of scientific progress. Whilst professionals have learnt lessons from history, the public may still have anxieties about the purpose of genetic services. There is an extensive regulatory and advisory framework for biotechnology in the UK. The bodies that have particular Genetic counselling 11 Box 3.5 Prenatal detection of unexpected abnormalities • serum biochemical screening • routine ultrasonography • amniocentesis for chromosomal analysis following abnormalities on biochemical or ultrasound screening Prenatal diagnosis of abnormalities anticipated prior to pregnancy • ultrasonography for known risk of specific congenital abnormality because of a previous affected child • chromosomal analysis because of familial chromosome translocation or previous affected child • molecular testing because of family history of single gene disorder Decision by patient to proceed with test timing agreed with patient • • Result session • Results communicated • Arrangements for follow up confirmed • Prompt written confirmation of result to patient and GP (if consent given) Follow-up Mutation positive result • Input from genetic service and primary care • Planned medical surveillance anticipating future onset of symptoms • Psychological support • Genetic counselling for children at appropriate age • Psychological support often needed towards adjustment and impact on relatives still at risk. Period of 2–6 weeks Test session • Written consent (including disclosure to GP) • Clear arrangements for result giving and follow up Mutation negative result Figure 3.4 Protocol for presymptomatic testing for late onset disorders Figure 3.5 Advances in genetic technology generate debate about ethical issues
  21. 21. responsibility for clinical genetics are the Human Genetics Commission, the Gene Therapy Advisory Committee and the Genetics and the Insurance Committee. Informed consent Competent adults can give informed consent for a procedure when they have been given appropriate information by professionals and have had the chance to think about it. With regard to genetic tests, the information given needs to include the reason for the test (diagnostic or predictive), its accuracy and the implications of the result. It may be difficult to ensure that consent is truly informed when the patient is a child, or other vulnerable person, such as an individual with cognitive impairment. This is of most concern if the proposed genetic test is being carried out for the benefit of other members of the family who wish to have a genetic disorder confirmed in order to have their own risk assessed. Genetic tests in childhood In the UK the professional consensus is that a predictive genetic test should be carried out in childhood only when it is in the best interests of the child concerned. It is important to note that both medical and non-medical issues need to be considered when the child’s best interests are being assessed. There may be a potential for conflict between the parents’ “need to know” and the child’s right to make his or her own decisions on reaching adulthood. In most cases, genetic counselling helps to resolve such situations without predictive genetic testing being carried out during childhood, since genetic tests for carrier state in autosomal recessive disorders only become of consequence at reproductive age, and physical examination to exclude the presence of clinical signs usually avoids the need for predictive genetic testing for late-onset dominant disorders. Confidentiality Confidentiality is not an absolute right. It may be breached, for example, if there is a risk of serious harm to others. In practice, however, it can be difficult to assess what constitutes serious harm. There is the potential for conflict between an individual’s right to privacy and his or her genetic relatives’ right to know information of relevance to themselves. Occasionally patients are reluctant to disclose a genetic diagnosis to other family members. In practice the individual’s sense of responsibility to his or her relatives means that, in time, important information is shared within most families. There may also be conflict between an individual’s right to privacy and the interests of other third parties, for example employers and insurance companies. Unsolicited information Problems may arise where unsolicited information becomes available. Non-paternity may be revealed either as a result of a genetic test, or through discussion with another family member. Where this would change the individual’s genetic risk, the professional needs to consider whether to divulge this information and to whom. In other situations a genetic test, such as chromosomal analysis of an amniocentesis sample for Down syndrome, may reveal an abnormality other than the one being tested for. If this possibility is known before testing, it should be explained to the person being tested. Non-directiveness Non-directiveness is taken to be a cornerstone of contemporary genetic counselling practice, and is important in promoting ABC of Clinical Genetics 12 A Fig. 3.6 Individual A was found to carry a balanced chromosomal rearrangement following termination of pregnancy for fetal abnormality. Initially she refused to inform her sister and brother about the potential risks to their future children, but decided to share this information when her sister became pregnant, so that she could have the opportunity to ask for tests Box 3.6 The Human Genetics Commission (HGC) • This is a strategic body that reports and advises on genetic technologies and their impact on humans. • The HGC has taken over the work of the Advisory Committee on Genetic Testing (AGCT), the Advisory Group on Scientific Advances in Genetics (AGSAG) and the Human Genetics Advisory Commission (HGAC). The Gene Therapy Advisory Committee (GTAC) • The GTAC reports and advises on developments in gene therapy research and their implications. • It also reviews and approves appropriate protocols for gene therapy research. The Genetics and Insurance Committee (GAIC) • This body reports and advises on the evaluation of specific genetic tests, including the reliability and relevance to particular types of insurance. • The GAIC reviews proposals from insurance providers and the degree of compliance by the insurance industry with the recommendations of the committee. 1 2 1 1 2 1 1 2 2 1 2 2 2 3 2 2 3 2 2 1 3 1 3 2 Figure 3.7 Genotyping using DNA markers linked to the SMN gene, undertaken to enable future prenatal diagnosis of spinal muscular atrophy, demonstrated that the affected child had inherited genetic markers not present in her father or mother, indicating non-paternity and affecting the risk of recurrence for future pregnancies.
  22. 22. autonomy of the individual. It is important for professionals to be aware that it may be difficult to be non-directive in certain situations, particularly where individuals or couples ask directly for advice. In general, genetic counsellors refrain from directing patients who are making reproductive or predictive test decisions, but there is an ongoing debate about whether it is possible for a professional to be non-directive, and whether such an approach is always appropriate for all types of decisions that need to be made by people with a family history of genetic disease. Genetic counselling 13
  23. 23. 14 The correct chromosome complement in humans was established in 1956, and the first chromosomal disorders (Down, Turner, and Klinefelter syndromes) were defined in 1959. Since then, refinements in techniques of preparing and examining samples have led to the description of hundreds of disorders that are due to chromosomal abnormalities. Cell division Most human somatic cells are diploid (2nϭ46), contain two copies of the genome and divide by mitosis. Germline oocytes and spermatocytes divide by meiosis to produce haploid gametes (nϭ23). Some human somatic cells, for example giant megakaryocytes, are polyploid and others, for example muscle cells, contain multiple diploid nuclei as a result of cell fusion. During cell division the DNA of the chromosomes becomes highly condensed and they become visible under the light microscope as structures containing two chromatids joined together by a single centromere. This structure is essential for segregation of the chromosomes during cell division and chromosomes without centromeres are lost from the cell. Chromosomes replicate themselves during the cell cycle which consists of a short M phase during which mitosis occurs, and a longer interphase. During interphase there is a G1 gap phase, an S phase when DNA synthesis occurs and a G2 gap phase. The stages of mitosis – prophase, prometaphase, metaphase, anaphase and telophase – are followed by cytokinesis when the cytoplasm divides to give two daughter cells. The process of mitosis produces two identical diploid daughter cells. Meiosis is also preceded by a single round of DNA synthesis, but this is followed by two cell divisions to produce the haploid gametes. The first division involves the pairing and separation of maternal and paternal chromosome homologs during which exchange of chromosomal material takes place. This process of recombination separates groups of genes that were originally located on the same chromosome and gives rise to individual genetic variation. The second cell division is the same as in mitosis, but there are only 23 chromosomes at the start of division. During spermatogenesis, each spermatocyte produces four spermatozoa, but during oogenesis there is unequal division of the cytoplasm, giving rise to the first and second polar bodies with the production of only one large mature egg cell. 4 Chromosomal analysis Figure 4.1 Normal male chromosome constitution with idiograms demonstrating G banding pattern of each individual chromosome (courtesy of Dr Lorraine Gaunt and Helena Elliott, Regional Genetic Service, St Mary’s Hospital, Manchester) Figure 4.2 The process of meiosis in production of mature egg and sperm Diploid oocyte Meiosis I 1st polar body 2nd polar body Meiosis II Haploid sperm Haploid egg Diploid spermatocyte Figure 4.3 Mitosis DNA replication Spindle formation MITOSIS Cell division Daughter cells
  24. 24. Chromosomal analysis 15 Chromosomal analysis Chromosomal analysis is usually performed on white blood cell cultures. Other samples analysed on a routine basis include cultures of fibroblasts from skin biopsy samples, chorionic villi and amniocytes for prenatal diagnosis, and actively dividing bone marrow cells. The cell cultures are treated to arrest growth during metaphase or prometaphase when the chromosomes are visible. Until the 1970s, chromosomes could only be analysed on the basis of size and number. A variety of banding techniques are now possible and allow more precise identification of chromosomal rearrangements. The most commonly used is G-banding, in which the chromosomes are subjected to controlled trypsin digestion and stained with Giemsa to produce a specific pattern of light and dark bands for each chromosome. The chromosome constitution of a cell is referred to as its karyotype and there is an International System for Human Cytogenetic Nomenclature (ISCN) for describing abnormalities. The Paris convention in 1971 defined the terminology used in reporting karyotypes. The centromere is designated “cen” and the telomere (terminal structure of the chromosome) as “ter”. The short arm of each chromosome is designated “p” (petit) and the long arm “q” (queue). Each arm is subdivided into a number of bands and sub-bands depending on the resolution of the banding pattern achieved. High resolution cytogenetic techniques have permitted identification of small interstitial chromosome deletions in recognised disorders of previously unknown origin, such as Prader–Willi and Angelman syndromes. Deletions too small to be detected by microscopy may be amenable to diagnosis by molecular in situ hybridisation techniques. Karyotypes are reported in a standard format giving the total number of chromosomes first, followed by the sex chromosome constitution. All cell lines are described in mosaic abnormalities, indicating the frequency of each. Additional or missing chromosomes are indicated by ϩ or Ϫ for whole chromosomes, with an indication of the type of abnormality if there is a ring or marker chromosome. Structural rearrangements are described by in dicating the p or q arm and the band position of the break points. Figure 4.5 Simplified banding pattern of chromosome 12 3.3 3.2 3.1 2.31 1 2 p q 2.2 2.1 1.2 1.1 1 2 3.1 3.2 3.3 4 5 1.1 1.2 1.3 2 3 4.1 4.2 4.3 12 Figure 4.4 Meiosis Pairing and recombination MEIOSIS Division I Division II Gametes Table 4.1 Definitions Euploid Chromosome numbers are multiples of the haploid set (2n) Polyploid Chromosome numbers are greater than diploid (3n, triploid) Aneuploid Chromosome numbers are not exact multiples of the haploid set (2nϩ1 trisomy; 2nϪ1 monosomy) Mosaic Presence of two different cell lines derived from one zygote (46XX/45X, Turner mosaic) Chimaera Presence of two different cell lines derived from fusion of two zygotes (46XX/46XY, true hermaphrodite)
  25. 25. ABC of Clinical Genetics 16 Molecular cytogenetics Fluorescence in situ hybridisation (FISH) is a recently developed molecular cytogenetic technique, involving hybridisation of a DNA probe to a metaphase chromosome spread. Single stranded probe DNA is fluorescently labelled using biotin and avidin and hybridised to the denatured DNA of intact chromosomes on a microscope slide. The resultant DNA binding can be seen directly using a fluorescence microscope. One application of FISH is in chromosome painting. This technique uses an array of specific DNA probes derived from a whole chromosome and causes the entire chromosome to fluoresce. This can be used to identify the chromosomal origin of structural rearrangements that cannot be defined by conventional cytogenetic techniques. Alternatively, a single DNA probe corresponding to a specific locus can be used. Hybridisation reveals fluorescent spots on each chromatid of the relative chromosome. This method is used to detect the presence or absence of specific DNA sequences and is useful in the diagnosis of syndromes caused by sub-microscopic deletions, such as William syndrome, or in identifying carriers of single gene defects due to large deletions, such as Duchenne muscular dystrophy. It is possible to use several separate DNA probes, each labelled with a different fluorochrome, to analyse more than Figure 4.6 47, XXX karyotype in triple X syndrome (courtesy of Dr Lorraine Gaunt and Helena Elliott, Regional Genetic Service, St Mary’s Hospital, Manchester) Table 4.2 Reporting of karyotypes Total number of chromosomes given first followed by sex chromosome constitution 46,XX Normal female 47,XXY Male with Klinefelter syndrome 47,XXX Female with triple X syndrome Additional or lost chromosomes indicated by ϩ or Ϫ 47,XY,ϩ21 Male with trisomy 21 (Down syndrome) 46,XX,12pϩ Additional unidentified material on short arm of chromosome 12 All cell lines present are shown for mosaics 46,XX/47,XX,ϩ21 Down syndrome mosaic 46,XX/47,XXX/45,X Turner/triple X syndrome mosaic Structural rearrangements are described, identifying p and q arms and location of abnormality 46,XY,del11(p13) Deletion of short arm of chromosome 11 at band 13 46,XX,t(X;7)(p21;q23) Translocation between chromosomes X and 7 with break points in respective chromosomes Figure 4.7 Types of chromosomal abnormality Type of disorder Triploidy Trisomy of chromosome 21 Monosomy of X chromosome 69 chromosomes Down syndrome Turner syndrome Cri du chat syndrome Klinefelter syndrome Associated with Wilms tumour Normal phenotype Infertility in females Mental retardation syndrome Mental retardation syndrome Balanced translocations cause no abnormality. Unbalanced translocations cause spontaneous abortions or syndromes of multiple physical and mental handicaps Lethal OutcomeExample 47 chromosomes (XXY) Terminal deletion 5p Interstitial deletion 11p Isochromosome X (fusion of long arms with loss of short arms) Ring chromosome 18 Fragile X ReciprocalTranslocation Fragile site Ring chromosome Duplication Inversion Deletion Structural Aneuploid Robertsonian q q 14 13 Pericentric inversion 9 Numerical Polyploid Figure 4.8 Fluorescence in situ hybridisation of normal metaphase chromosomes hybridised with chromosome 20 probes derived from the whole chromosome, which identify each individual chromosome 20 (courtesy of Dr Lorraine Gaunt, Regional Genetic Service, St Mary’s Hospital, Manchester)
  26. 26. Chromosomal analysis 17 one locus or chromosome region in the same reaction. Another application of this technique is in the study of interphase nuclei, which permits the study of non-dividing cells. Thus, rapid results can be obtained for the diagnosis or exclusion of Down syndrome in uncultured amniotic fluid samples using chromosome 21 specific probes. Incidence of chromosomal abnormalities Chromosomal abnormalities are particularly common in spontaneous abortions. At least 20% of all conceptions are estimated to be lost spontaneously, and about half of these are associated with a chromosomal abnormality, mainly autosomal trisomy. Cytogenetic studies of gametes have shown that 10% of spermatozoa and 25% of mature oocytes are chromosomally abnormal. Between 1 and 3% of all recognised conceptions are triploid. The extra haploid set is usually due to fertilisation of a single egg by two separate sperm. Very few triploid pregnancies continue to term and postnatal survival is not possible unless there is mosaicism with a normal cell line present as well. All autosomal monosomies and most autosomal trisomies are also lethal in early embryonic life. Trisomy 16, for example, is frequently detected in spontaneous first trimester abortuses, but never found in liveborn infants. In liveborn infants chromosomal abnormalities occur in about 9 per 1000 births. The incidence of unbalanced abnormalities affecting autosomes and sex chromosomes is about the same. The effect on the child depends on the type of abnormality. Balanced rearrangements usually have no phenotypic effect. Aneuploidy affecting the sex chromosomes is fairly frequent and the effect much less severe than in autosomal abnormalities. Unbalanced autosomal abnormalities cause disorders with multiple congenital malformations, almost invariably associated with mental retardation. Figure 4.9 Fluorescence in situ hybridisation of metaphase chromosomes from a male with 46 XX chromosome constitution hybridised with separate probes derived from both X and Y chromosomes. The X chromosome probe (yellow) has hybridised to both X chromosomes. The Y chromosome probe (red) has hybridised to one of the X chromosomes, which indicates that this chromosome carries Y chromosomal DNA, thus accounting for the subject’s phenotypic sex (courtesy of Dr Lorraine Gaunt, Regional Genetic Service, St Mary’s Hospital, Manchester) Table 4.3 Frequency of chromosomal abnormalities in spontaneous abortions and stillbirths (%) Spontaneous abortions All 50 Before 12 weeks 60 12–20 weeks 20 Stillbirths 5 Table 4.4 Frequency of chromosomal abnormalities in newborn infants (%) All 0.91 Autosomal trisomy 0.14 Autosomal rearrangements balanced 0.52 unbalanced 0.06 Sex chromosome abnormality 0.19
  27. 27. 18 Abnormalities of the autosomal chromosomes generally cause multiple congenital malformations and mental retardation. Children with more than one physical abnormality and developmental delay or learning disability should therefore undergo chromosomal analysis as part of their investigation. Chromosomal disorders are incurable but most can be reliably detected by prenatal diagnostic techniques. Amniocentesis or chorionic villus sampling should be offered to women whose pregnancies are at increased risk – namely, couples in whom one partner carries a balanced translocation, women identified by biochemical screening for Down syndrome and couples who already have an affected child. Unfortunately, when there is no history of previous abnormality the risk in many affected pregnancies cannot be predicted before the child is born. Down syndrome Down syndrome, due to trisomy 21, is the commonest autosomal trisomy with an overall incidence in liveborn infants of between 1 in 650 and 1 in 800. Most conceptions with trisomy 21 do not survive to term. Two thirds of conceptions miscarry by mid-trimester and one third of the remainder subsequently die in utero before term. The survival rate for liveborn infants is surprisingly high with 85% surviving into their 50s. Congenital heart defects remain the major cause of early mortality, but additional factors include other congenital malformations, respiratory infections and the increased risk of leukaemia. An increased risk of Down syndrome may be identified prenatally by serum biochemical screening tests or by detection of abnormalities by ultrasound scanning. Features indicating an increased risk of Down syndrome include increased first trimester nuchal translucency or thickening, structural heart defects and duodenal atresia. Less specific features include choroid plexus cysts, short femori and humeri, and echogenic bowel. In combination with other risk factors their presence indicates the need for diagnostic prenatal chromosome tests. The facial appearance at birth usually suggests the presence of the underlying chromosomal abnormality, but clinical diagnosis can be difficult, especially in premature babies, and should always be confirmed by cytogenetic analysis. In addition to the facial features, affected infants have brachycephaly, a short neck, single palmar creases, clinodactyly, wide gap between the first and second toes, and hypotonia. Older children are often described as being placid, affectionate and music-loving, but they display a wide range of behavioural and personality traits. Developmental delay becomes more apparent in the second year of life and most children have moderate learning disability, although the IQ can range from 20 to 85. Short stature is usual in older children and hearing loss and visual problems are common. The incidence of atlanto-axial instability, hypothyroidism and epilepsy is increased. After the age of 40 years, neuropathological changes of Alzheimer disease are almost invariable. Down syndrome risk Most cases of Down syndrome (90%) are due to nondisjunction of chromosome 21 arising during the first meiotic cell division in oogenesis. A small number of cases arise in meiosis II during oogenesis, during spermatogenesis or during mitotic cell division in the early zygote. Although occurring at any age, the 5 Common chromosomal disorders Figure 5.1 Child with dysmorphic facial features and developmental delay due to deletion of chromosome 18(18q-) Figure 5.2 Trisomy 21 (47, XX ϩ21) in Down syndrome (courtesy of Dr Lorraine Gaunt and Helena Elliott, Regional Genetic Service, St Mary’s Hospital, Manchester) Figure 5.3 Nondisjunction of chromosome 21 leading to Down syndrome Parents Gametes Offspring Non-viable Trisomy 21 Nondisjunction 2121
  28. 28. Common chromosomal disorders 19 risk of having a child with trisomy 21 increases with maternal age. This age-related risk has been recognised for a long time, but the underlying mechanism is not understood. The risk of recurrence for any chromosomal abnormality in a liveborn infant after the birth of a child with trisomy 21 is increased by about 1% above the population age related risk. The risk is probably 0.5% for trisomy 21 and 0.5% for other chromosomal abnormalities. This increase in risk is more significant for younger women. In women over the age of 35 the increase in risk related to the population age-related risk is less apparent. Population risk tables for Down syndrome and other trisomies have been derived from the incidence in livebirths and the detection rate at amniocentesis. Because of the natural loss of affected pregnancies, the risk for livebirths is less than the risk at the time of prenatal diagnosis. Although the majority of males with Down syndrome are infertile, affected females who become pregnant have a high risk (30–50%) of having a Down syndrome child. Translocation Down syndrome About 5% of cases of Down syndrome are due to translocation, in which chromosome 21 is translocated onto chromosome 14 or, occasionally, chromosome 22. In less than half of these cases one of the parents has a balanced version of the same translocation. A healthy adult with a balanced translocation has 45 chromosomes, and the affected child has 46 chromosomes, the extra chromosome 21 being present in the translocation form. The risk of Down syndrome in offspring is about 10% when the balanced translocation is carried by the mother and 2.5% when carried by the father. If neither parent has a balanced translocation, the chromosomal abnormality in an affected child represents a spontaneous, newly arising event, and the risk of recurrence is low (Ͻ1%). Recurrence due to parental gonadal mosaicism cannot be completely excluded. Occasionally, Down syndrome is due to a 21;21 translocation. Some of these cases are due to the formation of an isochromosome following the fusion of sister chromatids. In cases of true 21;21 Robertsonian translocation, a parent who carries the balanced translocation would be unable to have normal children (see figure 5.6). When a case of translocation Down syndrome occurs it is important to test other family members to identify all carriers of the translocation whose pregnancies would be at risk. Couples concerned about a family history of Down syndrome can have their chromosomes analysed from a sample of blood to exclude a balanced translocation if the karyotype of the affected person is not known. This usually avoids unnecessary amniocentesis during pregnancy. Figure 5.4 Down syndrome due to Robertsonian translocation between chromosomes 14 and 21 (courtesy of Dr Lorraine Gaunt and Helena Elliott, Regional Genetic Service, St Mary’s Hospital, Manchester) Figure 5.5 Possibilities for offspring of a 14;21 Robertsonian translocation carrier 21 14 21 14 Carrier of balanced translocation Non-viable Non-viable Non-viable Normal Balanced translocation Down syndrome Normal spouse Box 5.1 Risk of Down syndrome in livebirths and at amniocentesis Maternal age Liveborn Risk at (at delivery or risk amniocentesis amniocentesis) All ages 1 in 650 Age 30 1 in 900 Age 35 1 in 385 1 in 256 Age 36 1 in 305 1 in 200 Age 37 1 in 240 1 in 156 Age 38 1 in 190 1 in 123 Age 39 1 in 145 1 in 96 Age 40 1 in 110 1 in 75 Age 44 1 in 37 1 in 29 Figure 5.6 Possibilities for offspring of a 21;21 Robertsonian translocation carrier 2121 Normal spouse Parents Down syndrome in all offspring Carrier of balanced 21; 21 translocation Non-viable Gametes Offspring
  29. 29. ABC of Clinical Genetics 20 Other autosomal trisomies Trisomy 18 (Edwards syndrome) Trisomy 18 has an overall incidence of around 1 in 6000 live births. As with Down syndrome most cases are due to nondisjunction and the incidence increases with maternal age. The majority of trisomy 18 conceptions are lost spontaneously with only about 2.5% surviving to term. Many cases are now detectable by prenatal ultasound scanning because of a combination of intrauterine growth retardation, oligohydramnios or polyhydramnios and major malformations that indicate the need for amniocentesis. About one third of cases detected during the second trimester might survive to term. The main features of trisomy 18 include growth deficiency, characteristic facial appearance, clenched hands with overlapping digits, rocker bottom feet, cardiac defects, renal abnormalities, exomphalos, myelomeningocele, oesophageal atresia and radial defects. Ninety percent of affected infants die before the age of 6 months but 5% survive beyond the first year of life. All survivors have severe mental and physical disability. The risk of recurrence for any trisomy is probably about 1% above the population age-related risk. Recurrence risk is higher in cases due to a translocation where one of the parents is a carrier. Trisomy 13 (Patau syndrome) The incidence of trisomy 13 is about 1 per 15 000 live births. The majority of trisomy 13 conceptions spontaneously abort in early pregnancy. About 75% of cases are due to nondisjunction, and are associated with a similar overall risk for recurrent trisomy as in trisomy 18 and 21 cases. The remainder are translocation cases, usually involving 13;14 Robertsonian translocations. Of these, half arise de novo and half are inherited from a carrrier parent. The frequency of 13;14 translocations in the general population is around 1 in 1000 and the risk of a trisomic conception for a carrier parent appears to be around 1%. The risk of recurrence after the birth of an affected child is low but difficult to determine. Prenatal ultrasound scanning will detect abnormalities leading to a diagnosis in about 50% of cases. Most liveborn affected infants succumb within hours or weeks of delivery. Eighty percent die within 1 month, 3% survive to 6 months. The main features of trisomy 13 include structural abnormalities of the brain, particularly microcephaly and holoprosencephaly (a developmental defect of the forebrain), facial and eye abnormalities, cleft lip and palate, postaxial polydactyly, congenital heart defects, renal abnormalities, exomphalos and scalp defects. Survivors have very severe mental and physical disability, usually with associated epilepsy, blindness and deafness. Chromosomal mosaicism After fertilisation of a normal egg nondisjunction may occur during a mitotic division in the developing embryo giving rise to daughter cells that are trisomic and nulisomic for the chromosome involved in the disjunction error. The nulisomic cell would not be viable, but further cell division of the trisomic cell, along with those of the normal cells, leads to chromosomal mosaicism in the fetus. Alternatively a chromosome may be lost from a cell in an embryo that was trisomic for that chromosome at conception. Further division of this cell would lead to a population of cells with a normal karyotype, again resulting in mosaicism. In Down syndrome mosaicism, for example, one cell line has a normal constitution of 46 chromosomes and the other has a constitution of 47 ϩ 21. Figure 5.7 Trisomy 18 mosaicism can be associated with mild to moderate developmental delay without congenital malformations or obvious dysmorphic features Figure 5.8 Features of trisomy 13 include a) post-axial polydactyly b)scalp defects and c)mid-line cleft lip and palate (courtesy of Professor Dian Donnai, Regional Genetic Service, St Mary’s Hospital, Manchester) Figure 5.9 Mild facial dysmorphism in a girl with mosaic trisomy 21 a b c
  30. 30. Common chromosomal disorders 21 The proportion of each cell line varies among different tissues and this influences the phenotypic expression of the disorder. The severity of mosaic disorders is usually less than non-mosaic cases, but can vary from virtually normal to a phenotype indistinguishable from full trisomy. In subjects with mosaic chromosomal abnormalities the abnormal cell line may not be present in peripheral lymphocytes. In these cases, examination of cultured fibroblasts from a skin biopsy specimen is needed to confirm the diagnosis. The clinical effect of a mosaic abnormality detected prenatally is difficult to predict. Most cases of mosaicism for chromosome 20 detected at amniocentesis, for example, are not associated with fetal abnormality. The trisomic cell line is often confined to extra fetal tissues, with neonatal blood and fibroblast cultures revealing normal karyotypes in infants subsequently delivered at term. In some cases, however, a trisomic cell line is detected in the infant after birth and this may be associated with physical abnormalities or developmental delay. Mosaicism for a marker (small unidentified) chromosome carries a much smaller risk of causing mental retardation if familial, and therefore the parents need to be investigated before advice can be given. Chromosomal mosaicism detected in chorionic villus samples often reflects an abnormality confined to placental tissue that does not affect the fetus. Further analysis with amniocentesis or fetal blood sampling may be indicated together with detailed ultrasound scanning. Translocations Robertsonian translocations Robertsonian translocations occur when two of the acrocentric chromosomes (13, 14, 15, 21, or 22) become joined together. Balanced translocation carriers have 45 chromosomes but no significant loss of overall chromosomal material and they are almost always healthy. In unbalanced translocation karyotypes there are 46 chromosomes with trisomy for one of the chromosomes involved in the translocation. This may lead to spontaneous miscarriage (chromosomes 14, 15, and 22) or liveborn infants with trisomy (chromosomes 13 and 21). Unbalanced Robertsonian translocations may arise spontaneously or be inherited from a parent carrying a balanced translocation. (Translocation Down syndrome is discussed earlier in this chapter.) Reciprocal translocations Reciprocal translocations involve exchange of chromosomal segments between two different chromosomes, generated by the chromosomes breaking and rejoining incorrectly. Balanced reciprocal translocations are found in one in 500–1000 healthy people in the population. When an apparently balanced recriprocal translocation is detected at amniocentesis it is important to test the parents to see whether one of them carries the same translocation. If one parent is a carrier, the translocation in the fetus is unlikely to have any phenotypic effect. The situation is less certain if neither parent carries the translocation, since there is some risk of mental disability or physical effect associated with de novo translocations from loss or damage to the DNA that cannot be seen on chromosomal analysis. If the translocation disrupts an autosomal dominant or X linked gene, it may result in a specific disease phenotype. Once a translocation has been identified it is important to investigate relatives of that person to identify other carriers of Figure 5.10 Normal 8 month old infant born after trisomy 20 mosaicism detected in amniotic cells. Neonatal blood sample showed normal karyotype Figure 5.12 Balanced Robertsonian translocation affecting chromosomes 13 and 14 (courtesy of Dr Lorraine Gaunt and Helena Elliott, Regional Genetic Service, St Mary’s Hospital, Manchester) Figure 5.11 Balanced Robertsonian translocation affecting chromosomes 14 and 21 21 21 21 21 2114 14 14 14 Unbalanced Robertsonian translocation affecting chromosomes 14 and 21 and resulting in Down syndrome
  31. 31. ABC of Clinical Genetics 22 the balanced translocation whose offspring would be at risk. Abnormalities resulting from an unbalanced reciprocal translocation depend on the particular chromosomal fragments that are present in monosomic or trisomic form. Sometimes spontaneous abortion is inevitable; at other times a child with multiple abnormalities may be born alive. Clinical syndromes have been described due to imbalance of some specific chromosomal segments. This applies particularly to terminal chromosomal deletions. For other rearrangements, the likely effect can only be assessed from reports of similar cases in the literature. Prediction is never precise, since reciprocal translocations in unrelated individuals are unlikely to be identical at the molecular level and other factors may influence expression of the chromosomal imbalance. The risk of an unbalanced karyotype occurring in offspring depends on the individual translocation and can also be difficult to determine. An overall risk of 5–10% is often quoted. After the birth of one affected child, the recurrence risk is generally higher (5–30%). The risk of a liveborn affected child is less for families ascertained through a history of recurrent pregnancy loss where there have been no liveborn affected infants. Pregnancies at risk can be monitored with chorionic villus sampling or amniocentesis. Deletions Chromosomal deletions may arise de novo as well as resulting from unbalanced translocations. De novo deletions may affect the terminal part of the chromosome or an interstitial region. Recognisable syndromes have been delineated for the most commonly occurring deletions. The best known of these are cri du chat syndrome caused by a terminal deletion of the short arm of chromosome 5 (5p-) and Wolf–Hirschhorn syndrome caused by a terminal deletion of the short arm of chromosome 4 (4p-). Microdeletions Several genetic syndromes have now been identified by molecular cytogenetic techniques as being due to chromosomal deletions too small to be seen by conventional analysis. These are termed submicroscopic deletions or microdeletions and probably affect less than 4000 kilobases of DNA. A microdeletion may involve a single gene, or extend over several genes. The term contiguous gene syndrome is applied when several genes are affected, and in these disorders the features present may be determined by the extent of the deletion. The chromosomal location of a microdeletion may be initially identified by the presence of a larger visible cytogenetic deletion in a proportion of cases, as in Prader–Willi and Angelman syndrome, or by finding a chromosomal translocation in an affected individual, as occured in William syndrome. A microdeletion on chromosome 22q11 has been found in most cases of DiGeorge syndrome and velocardiofacial syndrome, and is also associated with certain types of isolated congenital heart disease. With an incidence of 8 per 1000 live births, congenital heart disease is one of the most common birth defects. The aetiology is usually unknown and it is therefore important to identify cases caused by 22q11 deletion. Isolated cardiac defects due to microdeletions of chromosome 22q11 often include outflow tract abnormalities. Deletions have been observed in both sporadic and familial cases and are responsible for about 30% of non-syndromic conotruncal malformations including interrupted aortic arch, truncus arteriosus and tetralogy of Fallot. Figure 5.13 Possibilities for offspring of a 7;11 reciprocal translocation carrier 7 11 7 11 Normal Balanced 7;11 translocation Parent with balanced 7;11 translocation Trisomy 7q Monosomy 11q Monosomy 7q Trisomy 11q Gametes Offspring Parents Figure 5.14 Cri du chat syndrome associated with deletion of short arm of chromosome 5 (courtesy of Dr Lorraine Gaunt and Helena Elliott, Regional Genetic Service, St Mary’s Hospital, Manchester) Figure 5.15 Fluorescence in situ hybridisation with a probe from the DiGeorge critical region of chromosome 22q11, which shows hybridisation to only one chromosome 22 (red signal), thus indicating that the other chromosome 22 is deleted in this region (courtesy of Dr Lorraine Gaunt and Helen Elliott, Regional Genetic Service, St Mary’s Hospital, Manchester)
  32. 32. Common chromosomal disorders 23 DiGeorge syndrome involves thymic aplasia, parathyroid hypoplasia, aortic arch and conotruncal anomalies, and characteristic facies due to defects of 3rd and 4th branchial arch development. Velocardiofacial syndrome was described as a separate clinical entity, but does share many features in common with DiGeorge syndrome. The features include mild mental retardation, short stature, cleft palate or speech defect from palatal dysfunction, prominent nose and congenital cardiac defects including ventricular septal defect, right sided aortic arch and tetralogy of Fallot. Sex chromosome abnormalities Numerical abnormalities of the sex chromosomes are fairly common and their effects are much less severe than those caused by autosomal abnormalities. Sex chromosome abnormalites are often detected coincidentally at amniocentesis or during investigation for infertility. Many cases are thought to cause no associated problems and to remain undiagnosed. The risk of recurrence after the birth of an affected child is very low. When more than one additional sex chromosome is present learning disability or physical abnormality is more likely. Turner syndrome Turner syndrome is caused by the loss of one X chromosome (usually paternal) in fetal cells, producing a female conceptus with 45 chromosomes. This results in early spontaneous loss of the fetus in over 95% of cases. Severely affected fetuses who survive to the second trimester can be detected by ultrasonography, which shows cystic hygroma, chylothorax, asictes and hydrops. Fetal mortality is very high in these cases. The incidence of Turner syndrome in liveborn female infants is 1 in 2500. Phenotypic abnormalities vary considerably but are usually mild. In some infants the only detectable abnormality is lymphoedema of the hands and feet. The most consistent features of the syndrome are short stature and infertility from streak gonads, but neck webbing, broad chest, cubitus valgus, coarctation of the aorta, renal anomalies and visual problems may also occur. Intelligence is usually within the normal range, but a few girls have educational or behavioural problems. Associations with autoimmune thyroiditis, hypertension, obesity and non-insulin dependent diabetes have been reported. Growth can be stimulated with androgens or growth hormone, and oestrogen replacement treatment is necessary for pubertal development. A proportion of girls with Turner syndrome have a mosaic 46XX/45X karyotype and some of these have normal gonadal development and are fertile, although they have an increased risk of early miscarriage and of premature ovarian failure. Other X chromosomal abnormalities including deletions or rearrangements can also result in Turner syndrome. Triple X syndrome The triple X syndrome occurs with an incidence of 1 in 1200 liveborn female infants and is often a coincidental finding. The additional chromosome usually arises by a nondisjunction error in maternal meiosis I. Apart from being taller than average, affected girls are physically normal. Educational problems are encountered more often in this group than in the other types of sex chromosome abnormalities. Mild delay with early motor and language development is fairly common and deficits in both receptive and expressive language persist into adolescence and adulthood. Mean intelligence quotient is often about 20 points lower than that in siblings and many girls require remedial teaching although the majority attend mainstream Figure 5.17 Lymphoedema of the feet may be the only manifestation of Turner syndrome in newborn infant Figure 5.18 Normal appearance and development in 3-year old girl with triple X syndrome Box 5.2 Examples of syndromes associated with microdeletions Syndrome Chromosomal deletion DiGeorge 22q11 Velocardiofacial 22q11 Prader–Willi 15q11-13 Angelman 15q11-13 William 7q11 Miller–Dieker (lissencephaly) 17p13 WAGR (Wilms tumour ϩ aniridia) 11p13 Rubinstein–Taybi 16p13 Alagille 20p12 Trichorhinophalangeal 8q24 Smith–Magenis 17p11 Figure 5.16 Cystic hygroma in Turner syndrome detected by ultrasonography (courtesy of Dr Sylvia Rimmer, Department of Radiology, St. Mary’s Hospital, Manchester)
  33. 33. Figure 5.19 47, XXY karyotype in Klinefelter syndrome (courtesy of Dr Lorraine Gaunt and Helena Elliott, Regional Genetic Service, St. Mary’s Hospital Manchester) Figure 5.20 Nondisjunction error at paternal meiosis II resulting in XYY syndrome in offspring XYXX XYY Parents Gametes Offspring Mother Father Meiosis I Meiosis II Fertilisation ABC of Clinical Genetics 24 schools. The incidence of mild psychological disturbances may be increased. Occasional menstrual problems are reported, but most triple X females are fertile and have normal offspring. Early menopause from premature ovarian failure may occur. Klinefelter syndrome The XXY karyotype of Klinefelter syndrome occurs with an incidence of 1 in 600 liveborn males. It arises by nondisjunction and the additional X chromosome is equally likely to be maternally or paternally derived. There is no increased early pregnancy loss associated with this karyotype. Many cases are never diagnosed. The primary feature of the syndrome is hypogonadism. Pubertal development usually starts spontaneously, but testicular size decreases from mid-puberty and hypogonadism develops. Testosterone replacement is usually required and affected males are infertile. Poor facial hair growth is an almost constant finding. Tall stature is usual and gynaecomastia may occur. The risk of cancer of the breast is increased compared to XY males. Intelligence is generally within the normal range but may be 10–15 points lower than siblings. Educational difficultes are fairly common and behavioural disturbances are likely to be associated with exposure to stressful environments. Shyness, immaturity and frustration tend to improve with testosterone replacement therapy. XYY syndrome The XYY syndrome occurs in about 1 per 1000 liveborn male infants, due to nondisjunction at paternal meiosis II. Fetal loss rate is very low. The majority of males with this karyotype have no evidence of clinical abnormality and remain undiagnosed. Accelerated growth in early childhood is common, leading to tall stature, but there are no other physical manifestations of the condition apart from the occasional reports of severe acne. Intelligence is usually within the normal range but may be about 10 points lower than in siblings and learning difficulties may require additional input at school. Behavioural problems can include hyperactivity, distractability and impulsiveness. Although initially found to be more prevalent among inmates of high security institutions, the syndrome is much less strongly associated with aggressive behaviour than previously thought although there is an increase in the risk of social maladjustment.

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